Rothwell, Gar W. , Crepet, William , Stockey, Ruth A. .
Is the anthophyte hypothesis alive and well? New evidence from bennettitalean seeds and cones.
The Bennettitales is an important group of Mesozoic fossil seed plants with large, mostly pinnate leaves, paracytic stomata, and either mono- or bisporangiate “flower-like” cones that has been repeatedly implicated in the origin of flowering plants. Seeds are orthotropous and terminate stalks that are interspersed with interseminal scales on a receptacle that is often enclosed by subtending bracts. For nearly a century, these distinctive characteristics and others have been interpreted by some scientists as providing support for the hypothesis of Arber and Parkin that Bennettitales forms a monophyletic group with Gnetales and flowering plants. Named the Anthophyte Hypothesis by Doyle and Donoghue, this view considers the bisporangiate bennettitalean cone structure, superficially evocative of flowers of Magnoliales, to be homologous with magnolioid flowers and transitional to Gnetales + angiosperms. A reevaluation of the seed and cone structure based on superbly preserved permineralized specimens reveals that bennettitalean seeds and associated interseminal scales are morphologically novel relative to either gnetalean seeds and their accessory parts, or to the seeds and enclosing structures of flowering plants. Whereas many systematic analyses of fossil and living seed plants support an anthophyte tree topology, there is no evidence from fossil bennettitaleans that any of their ovule-associated structures can be convincingly homologized with the outer integument or carpel of flowering plants or with the bracteoles surrounding the seeds in Gnetales. This suggests independent origins for seed enclosing organs in Bennettitales, Gnetales, and flowering plants, and invites a closer look at the validity of the anthophyte hypothesis, which already has been called to question by contravening analyses of nucleotide sequence data.
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1 - Ohio University, Department of Environmental & Plant Biology, Porter Hall, Richland Avenue, Athens, Ohio, 45701-2979, USA
2 - Cornell University, Department of Plant Biology, 228 Plant Science Building, Ithaca, New York, 14853, USA
3 - University of Alberta, Department of Biological Sciences, Biological Sciences Centre, Cw 405, Edmonton, Alberta, T6G 2E9, Canada
seed plant phylogeny.
Presentation Type: Oral Paper:Papers for Sections
Date: Tuesday, July 29th, 2008
Time: 9:30 AM